1. Field of the Invention
This invention relates to an objective lens system which is disposed at the tip of an endoscope.
2. Description of the Related Art
As an objective lens system for an endoscope, various lens systems have been developed. Since being disposed at the tip of the endoscope which is inserted into a human body or a fine space, the objective lens system must be very small in size.
In such a small size objective lens system, it is important that the lateral color aberration is excellent. As an objective lens system excellent in the lateral color aberration, there has been known one disclosed our Japanese Unexamined Patent Publication No. 2(1990)-188709.
The objective lens system for an endoscope disclosed in the patent publication comprises first to fifth lens elements L.sub.1 to L.sub.5 and an aperture stop is disposed between the second and third lens elements L.sub.2 and L.sub.3 as shown in FIG. 5.
Since the objective lens system for an endoscope must be very small in size, high technique is required to process the lens elements and to assemble them into an objective lens system, which takes large labor and a long time. Accordingly the objective lens system for an endoscope is much higher in cost than other lens systems.
Especially in the objective lens system shown in FIG. 5, an aperture stop in the form of a flat plate is disposed between the opposed curved surfaces of the second and third lens elements L.sub.2 and L.sub.3. Since the optical elements in the lens system are very small in size as described above, it is very difficult to position the flat plate with high accuracy, which adds to manufacturing cost of the lens system.